LED resistor calculator
Series resistor for any LED: value, nearest standard part, actual current, and power — instantly.
The circuit this computes
LED Indicator — fully explained →How it works
The resistor sees whatever voltage the LED doesn't take: V = Vs − Vf. Ohm's law turns that into a value: R = (Vs − Vf) / I. A red LED (Vf ≈ 2 V) on 5 V at 10 mA: R = 3 V / 0.01 A = 300 Ω.
Forward voltage depends on the LED's colour chemistry, not its size: red/yellow sit near 2 V, green/blue/white near 3–3.3 V. When in doubt, the datasheet's Vf at your chosen current is the number to use.
Common questions
An LED is not a resistor — its current rises almost vertically once the supply passes its forward voltage. Without something to limit current, the LED takes whatever the supply can deliver and burns out. The series resistor absorbs the difference between supply and forward voltage at a current you choose.
Modern indicator LEDs are bright at 2–5 mA; 10 mA is plenty for almost anything, and 20 mA is the traditional absolute maximum for small LEDs. Lower current = longer life, less heat, dimmer light.
A slightly larger resistor gives slightly less current — a marginally dimmer LED. A slightly smaller one gives more current, which eats into the LED's safety margin. Dimmer beats fried, so this calculator rounds up to the next E24 value.